In fact, the Scooter Libby case provided a good example of why - recall that, if I remember correctly, Libby was acquitted on the count of lying to the FBI that related to an interview that was not transcribed in contemporaneous notes produced at trial, even though the jury convicted him of what was apparently the same general statements to the grand jury. And there were signs that the jury simply didn't believe that Fiztgerald had proven beyond a reasonable doubt what Libby said in that interview.

Any lawyer who has ever gone back to a deposition or court transcript - or even a legal brief - knows that you sometimes come away recalling that something was said that wasn't, at least in so many words. That's why transcripts are invaluable. And it's why recorded interviews or videotaped confessions are, when practicable, a tremendous step forward for the system.

There's no valid investigatory reason for them to not testify on the record, but then there's no valid investigatory reason for them to testify, period; both the desire to get them to testify and the resistance are pure matters of power politics having nothing to do with the search for truth that's at stake in the judicial process.

Congressional hearings have nothing to do with the judicial process. However, it's pretty clear that Congress has the authority to hold hearings, to issue subpoenas, and to demand testimony under oath. Since the hearing involves the department of justice, they it is indeed the job of Congress to provide such a process. And under no circumstance should Executive Priviledge exist to circumvent the law. Bush has the right to get confidential advice, but if that advice is illegal, than that right goes by the boards. Had Congress not been an oval office shill for 6 years, and done what they swore to do (hah!), then we might not have had to go as far as we did.